Gas pump rating
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gas pump rating</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for octane that I've seen before include "One cat with hydrocarbon compound occurring in petrol" , "Fuel component in McLaren vehicle" , "Petrol quality rating" , "Flammable hydrocarbon - at once (anag)" , "Hydrocarbon found in petrol - at once (anag)" .)